First, sister needs to take responsibility for training her dog and cleaning him messes.
Clean up needs to be done with an enzymatic pet cleaner (Simple Solution or Nature's Miracle are great products). To keep it low budget, make your sis pay for this--get the gallon size.
The dog needs to go out frequently and be monitored or crated at all times. He won't retreat to a corner to pee or lift a leg on the sofa if he is being watched. Use the umbilical leash method when he is out in the house. Put his leash on and clip the other end to your belt loop. He can't wander and you will be able to monitor him. (Again, I'd tie him to your sister.)
Praise the dog when he does his business outside in an appropriate place. And while working on his training, he may need to go out every 1 to 2 hours at first. If this is an adult dog, he probably won't soil his crate. When he can't be monitored, put him in the crate.
I'm assuming this is a male dog. Neutering can help, but some neutered dogs will still mark. It depends on how ingrained the behavior is. Clean every place meticulously and monitoring him so he doesn't do it will help.
I don't advocate this, but you can get an item called a belly band or a male dog wrap. You wrap it around the dog's waist, covering the penis, and if the dog tries to mark, the urine is contained. You can use feminine pads to soak the urine. This will help cut down on the messes to clean, but the dog may suffer skin breakdown and will smell badly. Marking behavior will leave small amounts of urine behind. It is important to know if he is urinating inside (large amount of urine) or marking (small amounts of urine).
And since this is your home, you get to set the rules. If your sister isn't willing to work to correct this problem, you may need to let her know the dog can't continue to live here. Only you know your family's circumstances and what is appropriate. Good luck.